1. Field of the Invention
The invention relates to a method for source-controlled channel decoding by expanding the Viterbi algorithm incorporating a priori and a posteriori information on the source symbols.
2. Description of the Prior Art
In the digital transmission of source signal, such as speech, sound, pictures, television, after a source coding, for example a pulse code modulation (PCM), a redundancy-reducing or irrelevance-reducing coding (i.e. MUSICAM), etc., a coding in the broadest sense is employed, for example in the form of
a) channel coding
b) coded modulation
c) crosstalk, for example by multipath propagation at channel.
In most cases, the transmission methods set forth above under a) to c) can be represented by a so-called "finite state" machine having .nu. states. Examples of this are convolutional codes, trellis-coded modulation methods, a multistage coded modulation, transmission channels with crosstalk, magnetic recording channels, and the like. The initial symbols are received disturbed after a transmission and the source data are to be detected by means of a receiver to reestablish the useful signal.
When using shift registers or the so-called finite state machine for this purpose advantageously the Viterbi algorithm (VA), a sequential decoding, the M algorithm or algorithms derived therefrom are employed. The algorithms and the apparatuses implemented therewith perform a "maximum-likelihood" (ML) estimation or a "maximum a posteriori" (MAP) estimation of the source data. Errors in such an estimation lead to errors in the source signals, particularly when significant symbols, for example the most significant bits in the PCM coding, are falsified.
The Viterbi algorithm is prior art and is used widely in the applications set forth above under a) to c). Furthermore, various expansions and implementation forms of the Viterbi algorithm exist, including the soft output Viterbi algorithm only (SOVA) which is described in patent specification DE 39 10 739, in which further publications on the Viterbi algorithm are mentioned.
The Viterbi algorithm used for a detection, i.e. for example for a demodulation, equalization, decoding, and the like, cannot in the present form employ any a priori or a posteriori knowledge of the pulse data sequence although such knowledge is frequently available, such as known bits or known bit frequencies, or can be obtained With the aid of methods as will be described in detail hereinafter. However, a Viterbi algorithm employed in the same manner as hitherto cannot use this information.